Report of the Rochechouart crater excursion by Jelle Pensaert, Master student at Gent University
A brief introduction to the impact crater of Rochechouart
Despite all the pretty girls and handsome boys in Rochechouart, the small-sized city in the province of Nouvelle Aquitaine has a lot more to offer to tourists. One of the major tourist attractions in this region is the impact crater of Rochechouart. It might not be obvious to many, but the Rochechouart and the surrounding region contains an awful lot of evidence for an impact that happened about 210 million years ago in a region that was already altered by the Variscan orogeny. Due to this impact, shock metamorphism mainly caused the formation of impact melt as it also triggered the brecciation and fractionation of the surrounding area up to 20 km away from the centre of the impact. The evidence of the impact crater roughly considers the consequences of shock metamorphism and the occurrence of impact melt rock, suevites and polymict breccia; which are all rock types created by an impact; in the region.
A first encounter with this peculiar past is at the quarry of Champagnac. Over here, the impact gave birth to impact breccia; which is a rock type consisting of angular clasts and a matrix; as well as impact veins; comprised of carbonate, silicate or amphibolite due to pressure dissolution. Folding occurred concurrently with brecciation, as a result of the impact. Furthermore, a fluid-like deformation due to the formation of impact melt is visible.
1-The quarry in Champagnac
2. The Gallo-roman therms near the village of Chassenon
Near the village of Chassenon, there is a chance to visit the Gallo-roman therms. If there is, do not waste the opportunity of dressing like a full-fledged Roman in this outstanding museum. Besides, once you take a leap through time, you should go further down the road into the heather. On first sight, nothing seems out of the ordinary. However, the place once was a Roman quarry, where suevites were exploited. Though, in the middle of the quarry a lens made out of impact melt was left untouched. This might be due to the differences in visible characteristics of the two rock types as the Romans might have liked building with uniform bricks better. The lens of impact melt is visible in the landscape as a hill overgrown by trees and bushes.
And if you are really keen on history, there is another quarry in the county of Chassenon. This quarry is located on the field of a local farmer, so please ask the owner before going. History has shown that fawn mistakenly fall into the open pit, making a deadly plunge. This cow cemetery is positioned on the border of shatter cones; rocks with striations; and overlying impact breccia. Both rock types indicate the occurrence of a meteor impact. On a side-note, if a shatter cone is desired for indoor or outdoor decoration, a basic search on the internet might be satisfying. Though, beware for imitates and stolen goods.
3. The quarry near Chassenon where countless fawn came to die gruesome deaths. Shatter cones are visible in the rocks.
Perhaps taking a walk sounds like a wonderful idea. Finding the perfect location for a nice afternoon stroll is like a walk in the park. As such, there is the area around Pressignac-La lande. Taking the unpaved roads leads you to the most astounding landscapes. While hiking through nature, encountering formations containing brecciated gneiss, polymict breccia, suevites and/or other rocks generated by the impact is not out of the question. If you do, be sure to take a look at this rare phenomenon.
4. Outcrop next to one of the many unpaved roads, containing gneiss and monolithic breccia.
Does the countryside no longer seem interesting? Maybe it is time to explore one of the many medieval villages or small towns. Once back among the living, make sure to note the houses and some streets are entirely made up of rock types found in the surrounding areas. These rock types do also contain traces of the past as they mainly are the impact melt rocks, suevites and polymict breccia previously spoken of.
Lastly, did this rural and medieval region truly melt your heart? Well, so did the meteor impact 210 million years ago with the region. Remnants of the impact melt can still be found in and around settlements. For instance, in Rochechouart both a yellow impact melt and a red impact melt can be observed at the foot of the castle. It is fascinating how the colour of the melt changes rapidly over such little distance. However, do not forget to set foot in the castle, this unique place will lift the blanket lingering over the mysterious history of the place.
5. The Rochechouart castle standing on top of both yellow impact melt and red impact melt.
Getting hungry yet? It is time to go and search for some food. There are a lot of taverns, pubs and restaurants serving exquisite dishes in the region. If your budget does not allow the expense of going to a restaurant or if you are a backpacker who would love to stay true to the ways of a true hitchhiker, there is the possibility to buy some food and drinks in a supermarket or you can get a meteor or a comet in one of the many local bakeries.
6. The list of bread sold in one of the local bakeries of Rochechouart.
Are you excited yet and thrilled for some more information of the region? Do not hesitate to contact Philippe Lambert on Lambertbdx@gmail.com.
If the region is not entirely what you are looking for or you still want more exciting sightseeing, San Francisco might do the job. Be sure to look for the man with flowers in his hair. He also knows a lot about meteor impacts.
7. Thomas, one of the researchers of the Rochechouart impact crater during the field trip of May 2018. By many he is also known as the man with flowers in his hair.
Report of the Rochechouart crater excursion by Marlies Wermersche, Master student at Gent University
We started with a presentation of what we would do in the upcoming days and an introduction of the Rochechouart crater and its area. About 210 Ma, a comet hit the earth. The crater is not visible any more due to erosion. Though breccias, gneisses and melt proof that there was a comet impact in this area.
In the quarry of Champagnac we saw a mafic gneiss intruded by a pink-orange diorite/apatite granite. These are cut by hydrothermal veins. The veins have an age of about 200 Ma and are probably post-impact and/or post-tectonic. The rocks had gone through two stages of metamorphism. First was the impact of the comet and second was due to tectonics of the Variscan orogeny. We are located about five to ten kilometres under ground zero of the crater. At the site, we could look closely to huge blocks of rock. XRF was applied on these rocks. Core SC18 was drilled at the quarry.
Figure 1:the quarry in Champagnac
In Chassenon we first looked at an old Roman quarry. Here we had a look inside the crater. We were standing on top of suevites and saw a polymict breccia consisting of dark gneiss, pink/orange granite and green melt (glass?). At this site 3 cores (SC1, SC 2 and SC3) were drilled with a maximum depth of 4.5 meter. There was no need to drill deeper because it would have directly hit the basement of the crater. The top of the core contains of suevite followed by impactoclastites, green glass in a fine-grained matrix.
In the archaeological museum Cassinomagus, we walked around and saw how the Romans used the stones to make a temple, an aqueduct, a theatre and a thermae, Roman baths. While listening to the recorded explanation, we looked around in the stones for shatter cones. Unfortunately, no one found one. But no worries, we would see one in the next stop.
Figure 2: students looking at the building stones of the thermae in the archaeological museum Cassinomagus.
Less than a kilometre to the south of the museum we walked through a field to an outcrop were we finally saw a shatter cone. Here, we are on the basement of the crater. We saw a contact between microgranite and gneiss in the outcrop. The shatter cone was in the gneiss. On top, we saw breccia. Here, 3 cores (SC4, SC5 and SC6) were drilled.
Figure 3: shatter cone in an outcrop in the neighbourhood of Chassenon
In the outcrop in Pressignac-La lande, we saw gneiss and monolithic breccia. And in Valette we saw matrix melt with a lot of clasts. Here, a 60-meter core was drilled. In Rochechouart we looked at the cores and did some XRF on them. Then the press came for a convention (you can read the article here: https://www.lepopulaire.fr/rochechouart/science/environnement/2018/05/18/le-site-de-lastrobleme-de-rochechouart-chassenon-attire-des-chercheurs-internationaux_12852264.html#refresh). When the press left, we went to the castle of Rochechouart. There we saw some breccia with digested clasts. When we went down the hill we came across an outcrop with yellow melt with breccia on top. Two holes were drilled around the castle. One at the castle (SC15) and one down the hill (SC14). Lastly, we went to Montoume-Videux. There we saw some red melt.
Hereby I want to thank Philippe Lambert for the tour and his explanations.
Report of the Rochechouart crater excursion by Quinten Van Gaever, Master student at Leuven University
“If one comes to the Rochechouart crater expecting a large, visible hole in the landscape, they will leave disappointed. After 200 million years of erosion, all the crater’s relief has long disappeared. Any evidence to be found is in the rocks of the area, where they haven’t eroded away. The intense pressures and temperatures that an impact like this creates, will greatly change the make-up of the underlying rocks (often referred to as the target). In the case of the Rochechouart region, the target rocks constituted of gneiss (heavily deformed rocks that were altered during the continental collision that also created the Central Massif) and granites (igneous rocks, formed during the same collision). The impact greatly altered the rocks and created characteristic lithologies for impact sites, that were the main reason Rochechouart was recognized as one to begin with.
Breccias are the result of rocks breaking under intense pressure and then being turned into solid rock again. They consist of larger chunks, called clasts, separated by a matrix of significantly smaller particles that unifies them into solid rock. Breccias can be either mono- or polymict, depending on whether multiple rock types were mixed during the break-up process. These will occur deeper down and farther away from the impact site.
Impact melt rocks are the result of the impact’s intense heat turning the target rocks (partially) into a molten state and then cooling again. Often these also contain clasts, which in this case are simply chunks of rocks that failed to melt entirely. As expected, these impactites, as they’re referred to, generally occur closer to the impact point.
The intermediate between the two is a breccia that contains droplets of impact melt, as well as regular pieces of rock. This is THE characteristic rock of an impact site, as breccias and melt rocks can be replicated by other geological processes, and it is referred to as suevite. As can be expected, it generally occurs between the two other types.
All these lithologies can be found, with varying degrees of prevalence, throughout the Rochechouart region. Since these represent the base of the crater, as the top has eroded away, the crater initially extended far beyond the current boundaries of the impact rocks”
Quinten Van Gaever.